New Yorkers have set up makeshift memorials near the site of the terror attack that left eight people dead on Tuesday. The city is still grappling with what is being called the deadliest terror attack since 9/11. City officials are urging residents to keep moving forward. And that means getting out to celebrate an annual New York City tradition, the marathon. Here's New York City Mayor Bill de Blasio at a press conference on Wednesday.

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BILL DE BLASIO: It will go on as scheduled. It will be an extraordinary event as it always is. It will be well protected as it always is. And we will take additional measures to ensure that.

MARTIN: The race is expected to attract over 50,000 runners, more than 2,000 - 2 million, rather, spectators. We are joined now by Raymond Kelly. He's a two-time New York City police commissioner. His second term began just months after the 9/11 attacks. Commissioner, thanks so much for being with us this morning.

RAYMOND KELLY: Good to be with you.

MARTIN: What's the key to bringing back a sense of security? - since it's clearly been shaken in New York - but a sense of security after a terror attack like the one we saw?

KELLY: Well, I think uniform presence is important. It's something that New York City has been doing extensively since 9/11 but certainly at the marathon. People who go, the spectators, will see an awful lot of police presence. And there are things, of course, that they won't see. There are plainclothes officers in the crowd. They'll be, you know, lots of K-9 teams. There's a block of vehicles. There's anti-sniper teams. New York City is fortunate to have the largest police department in the country by far. So the resources are available. It'll cost a lot of overtime. But as the department said earlier this week, the presence of police officers will be ramped up over what has been a very significant deployment of police officers in the past at the marathon.

MARTIN: I mean, this attack was on a bike path, you know, all along the West Side Highway, a view of the Hudson River. Pedestrians, bikers, families are there all the time. And the weapon was a vehicle, a truck. It's the ultimate soft target. How in the world do you go about preventing things like this?

KELLY: With great difficulty - ideally, you have some intelligence information that's going to indicate something like this will happen. But obviously, it's very, very difficult to get these days, particularly with the use of encryption. These apps are readily available. And we know now that this individual was, in fact, communicating with other people using Telegram - is the name of the - this particular app used, which apparently is very effective. So it's difficult to do.

The department always has a significant uniform presence at large gatherings, large events. Obviously, this was not that. It was a bike path that was easily entered by this vehicle. Hindsight is 20/20, you know? Should there be barriers that prevent the vehicles from being able to do that? Sure but difficult to do.

MARTIN: Yeah.

KELLY: By definition, a bike path or jogging path you want open. You want the easy access. You want people to be able to move freely. So it's a real dilemma. And I know the city officials will be looking at this issue. Bollards are something that has proved to be effective, but they're expensive.

MARTIN: But I hear you saying that really the only way to do it is to focus on the perpetrator, is to invest in efforts that help identify people who have been radicalized.

KELLY: Yeah.

MARTIN: But that's...

KELLY: But that is proving to be extremely difficult. As I say, the, you know - the use of encryption has made that very difficult for investigators to spot people before they commit these acts. So it's no easy task.

MARTIN: President Trump tweeted that the suspect in this attack should receive the death penalty. How does that impact the investigation having the president weigh in like that before the justice system has been able to adequately move through this case?

KELLY: Well, hopefully, it doesn't impact on the investigation. The two charges against this individual - one of the two charges makes him potentially eligible for the death penalty. But there's a long process before you get there. So it's, you know - it's not clear that that's going to happen. But hopefully, the investigators just continue to do the - you know, a deep dive, a very intensive investigation on this individual's background and what motivated him and when he get radicalized - those sorts of things.

MARTIN: Do you think the president should have exerted his opinion that way?

KELLY: Well, I mean, that's what we see all the time, tweets from the president. Hopefully, it doesn't impact on the outcome of this case.

MARTIN: Raymond Kelly, former commissioner of the New York City Police Department, thanks so much for your time.